The present invention relates to a rotary head assembly for a magnetic recording and reproducing device or apparatus and more particularly to a rotation transmission means or device such as a rotary head cylinder assembly for video tape recorders (to be referred to as "VTR" for brevity in this specification) which requires a high speed and highly accurate transmission of rotation.
The problems imposed on the mechanical system, especially on the rotary head cylinder assembly of VTR are in general how to correctly maintain the relative speed and position between the head and the tape and how to reproduce. When there exist the variations in relative speed between the tape and head, the time axis of reproduced signal varies, causing the variations in hue and saturation of reproduced images and the horizontally off center raster. It follows therefor that the variations in relative speed must be reduced to a minimum.
In general, the rotation transmission devices must satisfy the following requirements:
1. the deviations in rotational speed must be reduced to a minimum; and PA1 2. radial vibration must be reduced to a minimum.
However, the prior art rotary head assemblies are in general such that the upper cylinder is supported by a pair of spaced rolling contact bearings. As a result, the variation in amount of lubricant (which is in general grease) sealed in the rolling-contact bearings and the methods for sealing the lubricant are immediately reflected in the variations in torque. That is, the variations in load are increased with increase in amount of lubricant, but when the lubricant is not sufficient, rapid wear results. Furthermore, unsatisfactory running accuracy of rolling-contact bearings also causes the variations in torque.
There has been an increasing demand for VTR which is both handy and portable. To this end, the rotary head assemblies must be made compact in size. Then, the inertia effect of the rotary head cylinder is reduced so that the variations in torque and consequently the variations in rotational speed result.
Vibration of the rotary head assembly is also caused by unsatisfactory running accuracy of rolling-contact bearings. Excessive play between mating parts also causes vibration. Because of vibration, the center or axis of rotation is deviated, resulting in the variations in rotational speed. As described above, the variations in rotational speed adversely affect the quality of reproduced images in the case of the rotary head assembly for VTR.
The problems of vibration and variations in rotational speed arise unavoidably as long as the rolling-contact bearings are used. Therefore there have been made various attempts to overcome these problems. For instance in order to reduce the vibration of the upper cylinder, the component parts of rolling contact bearings and rotary head assembly are machined and assembled at a higher degree of accuracy, and the moment of inertia of the rotary shaft is increased. However, the reduction in size of the rotary head assembly and hence VTR results in the decrease in flywheel effect of the upper cylinder. As a result, running accuracy of rolling contact bearings becomes more and more an important factor which affects vibration and variations in rotational speed of the upper cylinder.
Because of the disadvantages of the rolling contact bearings, there have been made various attempts to use pneumatic bearings in the rotary head assemblies for VTR especially for industrial or professional use. However, the pneumatic bearings are disadvantageous in that it requires a pressurized air source; that is, an air compressor and a pneumatic control circuit which is very complicated in construction. Therefore, it is difficult at present to incorporate the pneumatic bearings in the rotary head assemblies for home VTR which must be made portable.
In addition to the problems of vibration and variations in rotational speed, the use of rolling contact bearings gives rise to another problem. That is, in order to provide portable VTR, the dimensions of VTR must be reduced as practically as possible. The height of portable VTR is dependent on the height of the rotary head assembly which in turn is dependent upon the distance between a pair of rolling contact bearings supporting the rotary shaft of the upper cylinder. Thus, as long as the rolling contact bearings are used in the rotary head assemblies in VTR, there exists a limit to the reduction in size of portable VTR.